Security update for xen (important)

This update for xen fixes the following issues:

• CVE-2017-12855: Premature clearing of GTF_writing / GTF_reading lead to
  potentially leaking sensitive information (XSA-230, bsc#1052686).
• CVE-2017-12135: Unbounded recursion in grant table code allowed a
  malicious guest to crash the host or potentially escalate
  privileges/leak information (XSA-226, bsc#1051787).
• CVE-2017-12137: Incorrectly-aligned updates to pagetables allowed for
  privilege escalation (XSA-227, bsc#1051788).
• CVE-2017-11334: The address_space_write_continue function in exec.c
  allowed local guest OS privileged users to cause a denial of service
  (out-of-bounds access and guest instance crash) by leveraging use of
  qemu_map_ram_ptr to access guest ram block area (bsc#1048920).
• CVE-2017-11434: The dhcp_decode function in slirp/bootp.c allowed local
  guest OS users to cause a denial of service (out-of-bounds read) via a
  crafted DHCP
  options string (bsc#1049578).
• CVE-2017-10664: qemu-nbd did not ignore SIGPIPE, which allowed remote
  attackers to cause a denial of service (daemon crash) by disconnecting
  during a server-to-client reply attempt (bsc#1046637).